This two-n-one Personal and Domestic Alarm is a security alarm that transforms an ordinary doorknob into a highly sensitive burglar alarm. In order to gain easy access to your home burglars will first try your doors to see if they are unlocked. If a burglar touches your outside doorknob this device will instantly emit a120 dB alarm to scare him away and alert you to the attempted entry. Unlike other alarms, the burglar will be stopped before he enters your home. Even if he is wearing gloves, the alarm will sound as soon as the outside doorknob is touched. No installation required: simply hang on the inside doorknob of any wooden door. Please note that this alarm will only work on wooden doors. This Personal and Domestic Alarm can also be used as a personal alarm also. Place the included metal chain around your wrist and plug it into the alarm and place the alarm in your hand, pocket, handbag. As soon as the metal chain is pulled out the alarm sound until the metal chain is plugged back in the unit. Requires 1 9v battery (that is not included). The alarm measures 4 1/4" x 2" x 1". Please note that this alarm does not work with plastic door knobs.